Aluminum base alloy



3,282,688 ALUMINUM BASE ALLOY Michael J. Pryor and Douglas S. Keir, Hamden, and Philip R. Sperry, North Haven, Conn, assignors to Olin Mathieson Chemical Corporation No Drawing. Filed Oct. 21, 1965, Ser. No. 500,262 4 Claims. (Cl. 75--138) This application is a continuation-in-part of United States patent application Serial No. 304,923, filed August 27, 1963, now US. Patent 3,240,629.

The present invention relates to an improved aluminum base alloy which may be advantageously utilized in a number of applications. In particular, the present invention resides in an improved aluminum-tin alloy which has particularly advantageous characteristics when utilized in primary cells of the dry type as the anode thereof, said alloy also serving as the container for the cell. The improved alloy of the present invention is also highly advantageous, inter alia, as a protective cladding or coating on other aluminum alloys or on other metals, such as steel, to furnish cathodic protection against corrosion of the core material when exposed to moisture or aqueous environments.

The metal zinc is extensively employed as anode in the construction of dry cells, for example, flashlight batteries. Numerous proposals have been made heretofore to substitute aluminum for zinc as the anode material in order to utilize the numerous advantageous properties of the aluminum, for example, aluminum generally attains a higher anodic efiiciency than zinc, and has much higher coulombic output per unit mass of anode metal consumed. In addition, aluminum enjoys a greater ease of fabrication to thin gauge and to formed dry cell battery cases. Still further, aluminum has a generally higher corrosion resistance when the batteryis on open circuit.

Dry cells containing aluminum, aluminum-zinc alloys or aluminum base alloys in general as the anode material have, however, suffered from numerous significant disadvantages, for example, such cells generally require a considerable elapsed time for the cell current to reach its steady maximum value, especially if the cell is kept on the shelf for extended periods of time. This elapsed time prevents the highly desirable quick response for current when the cell is placed in a circuit. In addition, aluminum and aluminum base alloys generally suffer from perforation of the cell wall either during service or during storage if, for example, the common halides are used as the electrolyte. Furthermore, composite alloys utilized to overcome the foregoing disadvantages are not entirely satisfactory and are more expensive.

Aliiminum, either alloyed or unalloyed, has been used as a coating or cladding material to protect steel against corrosion, particularly in outdoor atmospheric environments, where the aluminum shows greater durability than galvanized or zinc coated steels and it maintains a better appearance. Ordinary aluminum and its alloys afford some cathodic protection against corrosion of exposed portions of the steel, but is well-known that sometimes exposed areas of steel which are some distance removed from the coating, as for example, the sheared edges of aluminum coated sheet steel, will show some unsightly rusting, even though it is being protected against catastrophic corrosion. An aluminum alloy which would serve as a more active anode to protect over greater distances is required.

Aluminum and some of its alloys are similarly used as claddings or coatings to cathodically protect other, less corrosion-resistant alloys against corrosion byacting sacrificially as an anode. The aluminum alloy selected for cladding must have an electrode potential which is United States Patent 322,688 Patented Nov. 1, .1966

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more negative than the core alloy and which remains more negative during the passage of corrosion currents. However, it is sometimes found that a cladding-core combination that will protect under one condition of corrosive electrolyte, temperature and aeration will cease to protect if one of the conditions, e.g., temperature or ion concentration, is changed. Therefore, there is a need for cladding alloys which will remain active as anodes under a variety of conditions and yet, will have inherently good corrosion resistance when not coupled to a cathode.

It is, therefore, an object. of the present invention to provide an improved aluminum base alloy.

It is a further object of the present invention to provide an improved aluminum base alloy having many uses, for example, in primary cells of the dry type.

It is a further object of the present invention to provide an alloy as above which may be utilized as the anode in a dry cell and which also may serve as the container for the cell.

It is a particular object of the present invention to provide an improved aluminum base alloy which may be conveniently utilized in dry cells enjoying the natural benefits of aluminum as the anode material while overcoming the heretofore suffered disadvantages in the use of this material.

It is an additional object of the present invention to provide an aluminum base alloy as aforesaid which is capable of galvanic currents of the same order of magnitude as those produced by zinc and which is also capable of higher galvanic currents, if desired. f

It is a still further object to provide an aluminum alloy which will be active as an anode when used as a coating or cladding to cathodically protect other aluminum alloys or other metals, such as steel, against corrosion.

Further objects and advantages will appear hereinafter.

In accordance with the present invention it has now been found that the foregoing objects and advantages may be readily accomplished and an improved aluminum base alloy provided, said alloy consisting essentially of between 0.01 to 0.5% tin, wherein the tin is retained in solid solution in an amount of from 0.01 to 0.06%, and

the balance essentially aluminum.

The improved aluminum base alloy of the present invention contains tin in an amount from 0.01 to 0.5 percent and at least 90.0 percent aluminum, and preferably at least percent aluminum. The tin is retained in solid solution to the minimum degree, i.e., only in 'suflicient amount to yield the desired range of current density and preferably in the range of 0.02 to 0.05 percent, with the excess tin, or a suitable third ingredient being provided as taught in United States Patent 3,180,728 to improve uniformity of corrosion and to improve anodic efficiency.

The preferred manner of preparing the alloy of the present invention is to heat an aluminum tin alloy containing from 0.01 to 0.5 percent tin at a temperature from to 540 C. In this temperature range the solid solubility of tin in aluminum is from about 0.05 to less than 0.02 percent. The time of heating is fora sufficient period of time to precipitate any excess tin from solid solution and to develop the particulate form which produces maximum uniformity of attack and anodic elliciency. Generally, the heating period within the preferred temperature range is at least 15 minutes and may vary between 15 minutes and 48 hours. Longer times may be used but no particular advantage is thereby obtained. After the heating period, the sample may be cooled rapidly or slowly. For simplicity, this treatment may be termed heterogenization treatment.

If desired, the heterogenization treatment may be combined with fabrication procedures, such as hot working or annealing.

The heterogenization treatment may be preceded by a homogenization treatment, described in detail in United States Patent 3,180,728. The homogenization treatment is intended to retain the maximum amount of tin in solid solution, i.e., up to 0.1 percent, by heating at a temperature of around 620 C :20 C. and then rapidly cooling. The homogenization treatment also serves to give a uniform distribution of the tin throughout the alloy. The heterogenization treatment which follows serves to reduce the tin in solid solution to levels acceptable for dry cell application or for use as a cladding or coating of another aluminum alloy or some other metal.

It is within the scope of this invention to include a combination of moderating methods, i.e., alloying combined with heterogenizing heat treatment, in order to produce special mechanical or physical properties in the anode alloy for certain purposes.

The alloy of the present invention contains from 0.01 to 0.5 percent tin. The preferred tin content varies with the means used for control of the amaunt of tin remaining in solid solution within the broad limits of 0.01 to 0.06 percent. The preferred tin content in solid solution may be maintained by the use of other alloying additions which serve to reduce the solid solubility of tin in aluminum, i.e., no heterogenizing treatment may be used and amounts of tin in excess of 0.06 percent employed while still retaining preferred solid solution levels provided that the appropriate alloying addition is employed to reduce the solid solubility of tin in aluminum. Specific alloying additions will be elaborated upon hereinafter.

High purity aluminum may be employed; however, high purity aluminum is much less economical than commercial and lower purity compositions which generally fulfill the same requirements. Therefore, it is preferred in the present invention to utilize lower purity aluminum, i.e., an alloy containing from 0.001 to 0.3 percent silicon and from about 0.001 to 1.0 percent iron. This lower purity composition may be substituted for the high purity one without detriment to the electrochemical characteristics.

It should be further understood that the alloy of the present invention may contain in addition to the aluminum and tin and the impurities, other metal components. These addition-a1 components may be added to achieve particularly desirable results.

Generally, insoluble elements may be added to the alloy, i.e., elements which have less than 0.03 percent solid solubility in aluminum at 620 C. The total amount of these insoluble elements should preferably be no greater than 1.0 percent. Examples are iron, nickel and cobalt. These insoluble elements have very little or no significant effect on current output as they do not reduce the solid solubility of tin in aluminum, but they act as second phase particulate cathodes and large amounts ultimately reduce anodic efliciency by promoting local corrosion of the anode. I p

Soluble elements may be also added to the alloy, i.e., elements which have greater than 0.03 percent solid solubility in aluminum. The soluble elements may be considered either lattice expanders or lattice contractors, i.e., ternary addition elements which either expand or contract the aluminum lattice. Generally the lattice expanders stabilize tin in retained solid solution and permit high galvanic currents to be drawn from the alloy. Therefore, since it is necessary to moderate the high galvanic current of the alloy of the present invention when it is utilized as the anode for a dry cell, it is not desirable to utilize large amounts of lattice expanders unless they are required for other purposes, such as strengthening the alloy, improving the castability or increasing galvanic efficiency, in which case their effect upon galvanic current may be counteracted by heterogenization treatment or by other alloying additions. Typical lattice expanders include, for example, magnesium, zirconium, gallium, bismuth and indium.

Lattice contractors generally reject tin from solid solution and have a moderating effect on the galvanic current. Therefore, it is desirable to utilize these lattice contractors, especially where no heterogenization treatment is used and tin in excess of 0.06% is used. Naturally, the amount of lattice contractor will vary in each particular case, but generally an amount from 0.(05 to 4 percent is used. Typical lattice contractors and representative amounts thereof include, for example, zinc from about 0.01 to 1 percent, copper from about 0.01 to 1 percent, silicon from about 0.01 to 7 percent, manganese from about 0.01 to 1 percent, silver of from 0.01 to 1 percent and mixtures thereof.

Those elements most effective in moderating. overly aggressive cell currents and preferred amounts thereof are: silver in a least 0.1 percent; zinc in at least 0.02 percent; copper in at least 0.01 percent; and silicon in at least 0.1 percent.

In the dry cell prepared from the 'alloy of the present invention, any suitable cathode may be employed, for example, the conventional carbon or graphite cathodes may be utilized. These are usually used with a conventional cathodic depolarizer, such as manganese dioxide.

The various electrolytes suggested in the art for use in dry cells may be conveniently used in the dry cell, for example, the chloride paste electrolytes conventionally used in dry cells are suited for used in combination with the aluminum base alloy of the present invention. The chloride paste electrolytes when utilized in the dry cell surprisingly do not result in perforation of the cell wall either during service or during storage.

It is a further significant advantage of the present invention that When the alloy of the present invention is utilized for the anode material in a dry cell it is unnecessary to employ composite container materials to overcome the disadvantages of perforation of the cell container by localized corrosion. These composite container materials have been frequently employed heretofore in order to enable the use of an aluminum alloy as the anode. It is highly desirable to avoid the use of these composite materials especially because of the added cost of manufacture and still attain the advantages rendered therein. It has been found in accordance with the present invention that it is unnecessary to utilize composite container materials in view of the surprising advantages inherent in the aluminum base alloy of the present invention.

The present invention and the improvements attained thereby will be more readily apparent from a consideration of the following illustrative examples.

Example I This example describes representative preparation of aluminum alloys with tin contents of 0.02, 0.04, 0.08, 0.12 and 0.20 percent. The aluminum used was at least 99.995% pure to which pure tin was added and stirred in While the aluminum was in the molten condition. Each alloy was cast into a rectangular steel mold coated on the inside with pure lime.

In this example, a block 3 x 3 x 0.85 inches was cut from each ingot after the ingots had been homogenization heat treated for 16 hours as 620 C. and cooled in air. Each block was alternately cold rolled to give reductions in thickness of about 35 percent, followed by intermediate annealing for one hour at 500 C., until a final thickness of 0.060 inch was reached. Final heat treatment consisted of heating at 620 C. for 8 hours,quenching in Water, and heterogenizing by subsequent heating at 400 C. for 24 hours and quenching in water. For each of these alloys the tin content in solid solution was in the range of 0.01 to 0.06 percent.

The above example describes only one of many fabrica-' tion sequences which have been successfully used to produce rolled sheet, including the use of hotrolling to pro: duce a substantial reduction in thickness prior to the use of cold rolling, and also including the use of intermediate anneals to furnish the heterogenizing heat treatment.

Example II A test cell was prepared in order to test the galvanic characteristics of the foregoing alloys. The cell used consisted of square cm. of surface area of the desired aluminum alloy as anode and an equal area of steel as cathode, with a 0.1 N sodium chloride electrolyte, as described in an article in the Journal of the Electrochemical Society, volume 105, No. 11, starting at page 629 and also as described in United StatesPatents 3,180,728, 3,- 186,836 and 3,189,486. This representative cell demonstrates the dry cell behavior to be expected of the anode alloy in a wide variety of chloride electrolytes. This example also demonstrates the behavior of an aluminumcoated steel article, coated with the alloy of the present invention, in which some bare steel is exposed to an aqueous corrosion environment.

Example III A test cell was set up in accordance with Example II utilizing the alloys prepared in Example I. The cell with these alloys began to produce a current as soon as the external circuit was closed. Also, dissolution of the rolled anode alloys according to this invention occurred uniformly over the surface during the period of operation of the cell, and there was negligible local corrosion of the anode alloys during periods of open circuit. Comparatively, an aluminum-one percent zinc alloy failed to produce current as soon as the external circuit was closed, did not dissolve uniformly and attained significant local corrosion.

Example IV A dry cell was prepared using an aluminum-tin alloy containing 0.12 percent tin and normal iron and silicon impurities. The alloy was prepared in accordance with the teachings of Example I to insure the tin in solid solution in the preferred range. The same excellent results were obtained as in Example III.

Example V This example illustrates the use of an impurity element or an intentionally added alloying element to moderate the current output. In this case, aluminum alloys containing 0.12 percent tin and about 0.05 percent iron with the moderating element silicon varying in the range of 0.05 to 0.30 percent. Within this range, upon setting up the test cell of Example II, the number of coulombs passed in 48 hours declined from 500 at the lowest silicon content to 30 at the highest silicon content. All alloys had first been homogenized to place the maximum amount of tin in solid solution. Thus, it was proved that a low level of current output, suitable for use in a dry cell circuit, can be obtained by suitable choice of a third element in the aluminum-tin alloy. A further demonstration of this effect was found in a high purity aluminum-0.20 percent tin alloy with silver additions in the range of 0.01 to 0.09 percent. At the lowest silver content the coulombs passed in 48 hours amounted to 1000, whereas, at the highest silver content it was reduced to 100.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

What is claimed is:

1. An aluminum base alloy consisting essentially of between 0.01 and 0.5% tin, where the tin is retained in solid solution in an amount from 0.01 to 0.06%, balance essentially aluminum.

2. An alloy according to claim 1 wherein said alloy contains between 0.001 and 0.3% silicon and between contains from 0.005 to 4% of a lattice contractor which has greater than 0.03% solid solubility in aluminum.

References Cited by the Examiner UNITED STATES PATENTS 3,180,728 4/1965 Pryor et al. 138 3,186,836 6/1965 Pryor et al. 75-138 3,189,486 6/1965 Pryor et al. 104197 X DAVID L. RECK, Primary Examiner.

R. O. DEAN, Assistant Examiner. 

1. AN ALUMINUM BASE ALLOY CONSISTING ESSENTIALLY OF BETWEEN 0.01 AND 0.5% TIN, WHERE THE TIN IS RETAINED IN SOLID SOLUTION IN AN AMOUNT FROM 0.01 TO 0.06%, BALANCE ESSENTIALLY ALUMINUM. 